It is a list of all the installed packages in each appliance, detailing their version numbers. It can be really handy if you wish to check whether package abc is installed and which version it is. It can also give you hints on where the package comes from (if it's not listed then it comes from an upstream tarball, rather than being a deb package). Probably for many users it has no real purpose but for those of us who like to poke around under the hood, it's invaluable!